Describe the rate limiting in user facing documentation

docs/user/pings/index.md

@@ -79,6 +79,9 @@ See [Using the Experiments API](../experiments-api.md) on how to record experime
 ## Ping submission
 
 The pings that the Glean SDK generates are submitted to the Mozilla servers at specific paths, in order to provide additional metadata without the need to unpack the ping payload.
+
+> **Note**: To keep resource usage in check, ping submission is rate limited for a maximum of 10 pings every 60 seconds.
+
 A typical submission URL looks like
 
   `"<server-address>/submit/<application-id>/<doc-type>/<glean-schema-version>/<document-id>"`
@@ -102,6 +105,7 @@ A pre-defined set of headers is additionally sent along with the submitted ping:
 | `X-Client-Type` | `Glean` | Custom header to support handling of Glean pings in the legacy pipeline |
 | `X-Client-Version` | e.g. `0.40.0` | The Glean SDK version, sent as a custom header to support handling of Glean pings in the legacy pipeline |
 
+
 ## Defining foreground and background state
 
 These docs refer to application 'foreground' and 'background' state in several places.